Add EXT_multisampled_render_to_texture test
Currently we don't have any meaningful test in meas CI for ms-rtt: piglit has none and the GL CTS has 3, but none of those test whether the implicit multisampling resolve is happening.
This test draws a triangle with an without multisampling, and then draws the same triangle a third time with glFramebufferTexture2DMultisample(), and then uses a basic MSE function to check whether the result is multisampled.